The present invention relates to processes for the production of thermoplastic polymer yarns and more particularly relates to manufacturing processes which produce highly uniform thermoplastic polymer yarns.
In the production of many synthetic thermoplastic polymer yarns which are melt spun, at least one drawing step is either coupled with or performed as a separate step in the manufacturing process. The drawing of the yarns to impart orientation and thereby reduce residual elongation and increase tensile strength requires, in most cases, that the yarn be subjected to a significant tension.
Usually for such drawing, the yarn is advanced in a draw zone between sets of rotating rolls including feed rolls and subsequent draw rolls which are rotated at a higher peripheral speed than the feed rolls to impart the desired tension on the yarn. However, because the yarn which is advancing to the feed rolls is usually at a much lower tension than the tension in the draw zone, at least some slippage frequently occurs on the feed rolls and sometimes the draw begins to occur on the rolls. It has been discovered that this slippage and the drawing on the feed rolls can cause variations in the yarn and can affect the suitability of the yarns for end use applications such as in fabrics which are dyed with structure sensitive reactive dyes. When such dyes are used, otherwise undetectable variations in the yarns can be responsible for highly visible non-uniformity in fabric dyeing.
It has also been discovered that the decrease in tension across a roll set also can cause a non-uniform yarn to be produced due to slippage. This can occur in a wide variety of yarn processes when the tension on the yarn decreases substantially across a roll set. For example, when the yarn leaves a draw zone in a drawing step, tension is often substantially reduced. Similarly, in extremely high speed spinning process without a draw step in which the yarn is withdrawn from a spinneret at a speed to impart a high degree of spin orientation, the tension on the yarn can be very substantial and can be too high for the winding up of good packages. A tension let-down step may be necessary before windup.
Various techniques are known for preventing slippage on rolls when a tension change is occurring but known techniques generally introduce additional problems. For example, a pinch roll with a softer surface is known for use as disclosed in U.S. Pat. No. 2,413,073 to control tension induced slippage. However, the surface of such pinch roll wears to form grooves and the rolls therefore needs frequent maintenance. While a significant improvement in this apparatus can be obtained by traversing of the thread across the face of the roll and pinch roll contact as disclosed in GB 907,904, a wear problem still exists and the speed of such devices limits their use to only the slowest of processes. The use of aprons or belts to minimize slippage is useful as in Japanese Application No. 60-065,142-A, but great care must be taken to replace the belts when worn to prevent off-quality product.
It is also known to alter slippage by using a draw pin to lower the feed roll tension at the feed roll and isolate the draw at the pin as taught in U.S. Pat. No. 2,289,232. Rotating pins are also known as in U.S. Pat. No. 3,655,839. However, there is the potential for filament breakage and other mechanical damage when such devices are used.
Heated jets are sometimes used to reduce drawing tension and this minimizes tension induced slippage as in U.S. Pat. No. 3,452,130. However, such jets are not suitable for many types of processes and problems in controlling the uniformity of temperature and friction when these hot jets are used makes the process much more difficult to run and more expensive.
The use of snubbing pins to increase pre-tension is a useful route to minimize tension induced slippage as in U.S. Pat. No. 2,728,973 and U.S. Pat. No. 3,752,457 but can sometime also result in filament breakage.